Energy Scavenging for Wireless Sensor Networks: with Special Focus on Vibrations

The vast reduction in size and power consumption of CMOS circuitry has led to a large research effort based around the vision of ubiquitous networks of wireless communication nodes. The wireless devices are usually designed to run on batteries. However, as the networks increase in number and the devices decrease in size, the replacement of depleted batteries is not practical. Furthermore, a battery that is large enough to last the lifetime of the device would dominate the overall system size, and thus is not very attractive. There is clearly a need to explore alternative methods of powering these small communication nodes. This book, therefore, focuses on potential "ambient" sources of power that can be scavenged or harvested and subsequently used to run low power electronics and wireless transceivers. A wide range of potential power sources are briefly explored. Based on a comparison of these many potential sources, commonly occurring vibrations was chosen as an attractive, and little explored, power source. Models for different types of power converters using both electrostatic and piezoelectric conversion mechanisms have been developed. The models have been validated by testing prototypes driven at vibrations similar to those found in many industrial and commercial building environments. Finally, integration of a piezoelectric generator, power circuit, and custom design radio transceiver is demonstrated. Power sources are becoming a bottleneck to the widespread deployment of wireless sensor networks. This work reviews many potential alternative sources of ambient power that can be scavenged. Vibration to electricity converters are explored in great detail, and based on studies and experiments, are shown to be an attractive power source in many applications.

Table of Contents:
1 Introduction.- 1. Energy Storage.- 2. Power Distribution.- 3. Power scavenging.- 4. Summary of potential power sources.- 5. Overview of Vibration-to-Electricity Conversion Research.- 2 Vibration Sources and Conversion Model.- 1. Types of Vibrations Considered.- 2. Characteristics of Vibrations Measured.- 3. Generic Vibration-to-Electricity Conversion Model.- 4. Efficiency of Vibration-to-Electricity Conversion.- 3 Comparison of Methods.- 1. Electromagnetic (Inductive) Power Conversion.- 2. Electrostatic (Capacitive) Power Conversion.- 3. Piezoelectric Power Conversion.- 4. Comparison of Energy Density of Converters.- 5. Summary of Conversion Mechanisms.- 4 Piezoelectric Converter Design.- 1. Basic Design Configuration.- 2. Material Selection.- 3. Analytical Model for Piezoelectric Generators.- 4. Discussion of Analytical Model for Piezoelectric Generators.- 5. Initial Prototype and Model Verification.- 6. Design Optimization.- 7. Analytical Model Adjusted for a Capacitive Load.- 8. Discussion of Analytical Model Changes for Capacitive Load.- 9. Optimization for a Capacitive Load.- 10. Conclusions.- 5 Piezoelectric Converter Test Results.- 1. Implementation of Optimized Converters.- 2. Resistive load tests.- 3. Discussion of resistive load tests.- 4. Capacitive load tests.- 5. Discussion of capacitive load test.- 6. Results from testing with a custom designed RF transceiver.- 7. Discussion of results from custom RF transceiver test.- 8. Results from test of complete wireless sensor node.- 9. Discussion of results from complete wireless sensor node.- 10. Conclusions.- 6 Electrostatic Converter Design.- 1. Explanation of concept and principle of operation.- 2. Electrostatic Conversion Model.- 3. Exploration of design concepts and device specific models.- 4. Comparison ofdesign concepts.- 5. Design Optimization.- 6. Flexure design.- 7. Discussion of design and conclusions.- 7 Fabrication of Electrostatic Converters.- 1. Choice of process and wafer technology.- 2. Basic process flow.- 3. Specific processes used.- 4. Conclusions.- 8 Electrostatic Converter Test Results.- 1. Macro-scale prototype and results.- 2. Results from fluidic self-assembly process prototypes.- 3. Results from integrated process prototypes.- 4. Results from simplified custom process prototypes.- 5. Discussion of Results and Conclusions.- 9 Conclusions.- 1. Justification for focus on vibrations as a power source.- 2. Piezoelectric vibration to electricity converters.- 3. Design considerations for piezoelectric converters.- 4. Electrostatic vibration to electricity converters.- 5. Design considerations for electrostatic converters.- 6. Summary of conclusions.- 7. Recommendations for future work.- Acknowledgments.- Appendix A: Analytical Model of a Piezoelectric Generator.- 1. Geometric terms for bimorph mounted as a cantilever.- 2. Basic dynamic model of piezoelectric generator.- 3. Expressions of interest from basic dynamic model.- 4. Alterations to the basic dynamic model.- Appendix B: Analytical Model of an Electrostatic Generator.- 1. Derivation of electrical and geometric expressions.- 2. Derivation of mechanical dynamics and electrostatic forces.- 3. Simulation of the in-plane gap closing converter.- References.